In Fox et al., U.S. Pat. No. 4,507,442, it is disclosed that diprimary dialcohol polyesters, such as poly(ethylene arylates) and more specifically, poly(ethylene terephthalates) are not compatible with poly(bisphenol-A carbonates), but that by coreacting a blend or melt mix of the two, they can be compatibilized. However, it is essential to heat at 270.degree. C. under a high vacuum or in an inert atmosphere to obtain a copolymer having a single glass transition temperature. Moreover, while the copolymers obtained in '442 are eminently suitable for many uses, there is a tendency, in some instances, to develop a yellow color, particularly over a long time.
Okamura et al., U.S. Pat. No. 3,218,372, disclose molding compositions comprising polycarbonates and polyalkylene terephthalates, e.g., poly(bisphenol-A carbonate and poly(ethylene terephthalate) whereby the two polymers are melt-mixed under a nitrogen atmosphere. Although the compositions are used for molding, they are merely melted together under nitrogen, e.g., at 290.degree. C., and they are not described to be copolymers, but merely a "uniform mixture".
Schade et al., U.S. Pat. Nos. 3,299,172 and 3,413,379 disclose processes for the preparation of linear thermoplastic mixed polyesters by reacting a diaryl arylate, a poly(alkylene arylate) and, optionally, a diaryl carbonate in the presence of a transesterification and polycondensation catalyst. The materials produced by these processes are described to be "mixed polyesters" and they are not copolycopolymers containing poly(aryl carbonate) units, although isolated mono aryl carbonate linkages might be produced if a diaryl carbonate is included in the reaction mixture.
In Mercier et al., U.K. Patent No. 1,569,296, it is disclosed that mixing aromatic polyesters and aromatic polycarbonates in the molten state induces a reaction leading to fast degradation of the polymers, but if an acidic stabilizing additive is then added, a useful product can be obtained, but in which the polyester and the polycarbonate are only "partly copolymerized". The stabilizing additive serves to prevent further reaction and is therefore, not a catalyst. All of the additives exemplified in U.K. '296 are acidic, such as a phosphorus compound or a carboxylic acid. Moreover, such compositions will have three glass transition temperatures, one each for the respective homopolymers, and one for the copolymer. In terms of ultimate physical properties and appearance, as well as environmental resistance, such compositions are deficient, especially in comparison with the compositions of the Fox et al. patent which have only a single glass transition temperature. Such compositions also tend to become yellow with time.
In the current state of the art, therefore, it would be desirable to provide a convenient method to prepare novel low color co-polymers having a single glass transition temperature by melt extruding a polycarbonate and/or a polyester carbonate, a polyester and a catalyst, thus avoiding the need to use a subsequent step comprising high vacuum heating or heating in an inert atmosphere. Such novel copolymers and processes for their preparation are provided by the present invention.